The present invention is directed to a method and an apparatus for reducing the mean standby power consumption of battery operated cordless or radio telephones.
It is known in the prior art that mean power consumption of battery operated portable telephones can be reduced on standby mode, that is, during the time between conversations when waiting for the subscriber to initiate a new call or receive a call from the network. The principal mechanism to reduce standby power consumption is to turn off the transmitter so that only the receiver is operating, listening to a designated network calling channel. However, modern hand-held telephones are so small that the available capacity of the small internal batteries is only sufficient to run the receiver continuously for a few hours. Accordingly, another method has been described in the European Global System for Mobile Communications (GSM) digital cellular system standard to further reduce receiver standby power consumption. This method involves pulsing the receiver on and off with a low duty factor, the periods the receiver is on being known to the network in advance for each portable station so that the network can time the sending of messages and call a particular mobile subscriber at those instants.
Portable telephones are, according to the GSM system specification, divided into sleep-mode groups either according to some of the digits of their telephone numbers or as designated by the network operator. As a simple example, those telephones having a number ending in 0 could belong to sleep-mode group 0; those ending in 1 to sleep-mode group 1 and so on. The network calling channel capacity is divided in time into a number of cyclically repeating periods corresponding to the sleep-mode groups. A call to a telephone is then transmitted only in a period corresponding to its sleep-mode group, when it is known to be awake. Upon waking, receiving such a message from the network and detecting that its own telephone number or identification number (ID) is being called, the portable receiver may stop pulsing and remain on continuously to receive further information and the portable transmitter may be activated to reply to the call when the subscriber accepts the call by, for example, pressing an appropriate button.
One limitation of these conventional paging techniques is that a mobile station may miss a call from the network due to temporarily being in a radio shadow from the calling station. Limited radio capacity on the calling channel prevents the network from simply repeating the call indefinitely until answered, or even more than a couple of times. Another limitation of these conventional techniques is that the duty factor of the portable receiver can not be reduced indefinitely during standby due to the consequent delay in placing a call. A certain minimum time is required to transmit sufficient information to identify a particular mobile and the opportunities to call that mobile station arise N times less frequently where N is the number of sleep-mode groups or the reciprocal of the receiver standby duty factor. The minimum time required for transmission is typically at least 40 milliseconds (ms), while the opportunities to call the mobile station typically arise every 40.times.N ms. If N is 50 for example, up to 2 seconds delay is incurred in calling a portable telephone, and if typically two attempts are made, this delay can be as high 4 seconds. Greater delays than these are not desired by subscribers that lead a hectic daily life requiring prompt call placement.